U.S. patent application number 09/793748 was filed with the patent office on 2002-08-29 for magnetic media defect detection apparatus and method.
This patent application is currently assigned to Trace Storage Technology Corp.. Invention is credited to Yong, Philip.
Application Number | 20020118473 09/793748 |
Document ID | / |
Family ID | 25160688 |
Filed Date | 2002-08-29 |
United States Patent
Application |
20020118473 |
Kind Code |
A1 |
Yong, Philip |
August 29, 2002 |
Magnetic media defect detection apparatus and method
Abstract
Apparatus for detecting defects in a magnetic disk includes a
read head including multiple read elements for reading multiple
tracks of the disk. Read signals produced by the read elements are
received by certifier circuit portions for determining whether any
of the read signals includes an indication of a defect in the disk.
A monitoring circuit monitors the read elements to detect a
malfunctioning read element. The read head position is controlled
so that a malfunctioning read element is not used.
Inventors: |
Yong, Philip; (Hsinchu,
TW) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT &
DUNNER LLP
1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Assignee: |
Trace Storage Technology
Corp.
|
Family ID: |
25160688 |
Appl. No.: |
09/793748 |
Filed: |
February 27, 2001 |
Current U.S.
Class: |
360/31 ;
360/78.04; G9B/27.052; G9B/5.157; G9B/5.169 |
Current CPC
Class: |
G11B 2220/20 20130101;
G11B 5/4976 20130101; G11B 27/36 20130101; G01R 33/1207 20130101;
G11B 5/4886 20130101 |
Class at
Publication: |
360/31 ;
360/78.04 |
International
Class: |
G11B 027/36; G11B
005/596 |
Claims
What is claimed is:
1. Apparatus for reading a plurality of information tracks in a
magnetic storage medium, comprising: a read head including a
plurality of read elements each producing a read signal in response
to data stored in the tracks of the storage medium; and an element
monitoring circuit to monitor said plurality of elements and
provide an indication when one of said elements malfunctions.
2. The apparatus of claim 1 wherein each of said elements is a
magnetoresistance effect element; said monitoring circuit
comprising a comparator coupled to receive an element voltage
across one of said elements on a first input and a reference
voltage on a second input and provide the malfunction indication
when the element voltage changes relative to the reference voltage
in a predetermined way.
3. The apparatus of claim 2 wherein said reference voltage is a
programmable reference voltage.
4. The apparatus of claim 3 wherein the value of said programmable
reference voltage is determined by an average read signal of a
previously read track.
5. The apparatus of claim 1 further including: means for
positioning said read head over said storage medium; and means for
controlling said positioning means, said controlling means
responsive to the indication that one of the storage elements is
malfunctioning to position said read head to read predetermined
ones of the tracks of the storage medium without using the
malfunctioning element.
6. The apparatus of claim 5 further including means for determining
a step size for movements of said positioning means as a function
of which of said plurality of read elements is malfunctioning.
7. The apparatus of claim 5 wherein said controlling means controls
said positioning means to read substantially all tracks of the
storage medium; and means for determining a step size for movements
of said positioning means as a function of which of said plurality
of elements is malfunctioning, so that substantially all tracks of
the storage medium are read.
8. The apparatus of claim 7 wherein said magnetic storage medium is
a magnetic disk; and wherein said plurality of elements are mounted
in said read head adjacent to one another for positioning over
respective adjacent tracks of the magnetic disk.
9. Apparatus for reading a plurality of information tracks in a
magnetic storage medium, comprising: a read head including a
plurality of read elements each producing a read signal in response
to data stored in the storage medium; means for storing a plurality
of the read signals output by a selected one of said read elements;
and means for comparing at least one of the stored read signals
with at least one subsequent read signal output by said selected
read element, for determining whether said selected read element
malfunctions, and for providing an indication when said read
element malfunctions.
10. The apparatus of claim 9 wherein said storing means stores
successive values of the read signals output by said plurality of
read elements and said comparing means determines whether any of
said plurality of read elements malfunctions and provides the
indication when any of said read elements malfunctions; said
apparatus further including: means for positioning said read head
over said storage medium; and means for controlling said
positioning means, said controlling means responsive to the
indication that one of said elements is malfunctioning to position
said read head to read predetermined ones of the tracks of said
medium without using the malfunctioning element.
11. The apparatus of claim 10 further including means for
determining a step size for movements of said positioning means as
a function of which of said plurality of elements is
malfunctioning.
12. The apparatus of claim 10 wherein said controlling means
controls said positioning means to read substantially all tracks of
the storage medium; and means for determining a step size for
movements of said positioning means as a function of which of said
plurality of elements is malfunctioning, so that substantially all
tracks of the storage medium are read.
13. The apparatus of claim 12 wherein said magnetic storage medium
is a magnetic disk; and wherein said plurality of elements are
mounted in said read head adjacent to one another for positioning
over respective adjacent tracks of the magnetic disk.
14. A method for reading a plurality of information tracks in a
magnetic storage medium, comprising: producing read signals from a
plurality of read elements in response to data stored in the track
of the storage medium; monitoring the plurality of read elements to
determine when any one of the read elements malfunctions; and
providing an indication when any one of the read elements is
determined to be malfunctioning.
15. The method of claim 14 wherein monitoring includes comparing an
element voltage across any one of the read elements with a
reference voltage and determining the element is malfunctioning
when the element voltage changes relative to the reference voltage
in a predetermined way.
16. The method of claim 14 further including: positioning the read
head over the storage medium; and controlling the position of the
read head, in response to the indication that one of the read
elements is malfunctioning, to read predetermined ones of the
tracks of the storage medium without using the malfunctioning
element.
17. The method of claim 16 further including determining a step
size for movement of the read head as a function of which of the
plurality of read elements is malfunctioning.
18. The method of claim 16 wherein controlling the read head
position includes controlling the read head to read substantially
all tracks of the storage medium; the method further including
determining a step size for movements of the read head as a
function of which of the plurality of read elements is
malfunctioning, so that substantially all tracks of the storage
medium are read.
19. A method for reading a plurality of information tracks in a
magnetic storage medium, comprising: producing read signals from a
plurality of read elements in response to data stored in the
storage medium; storing ones of the read signals output by a
selected one of the read elements; comparing at least one of the
stored read signals with at least one subsequent read signal output
by the selected read element; determining whether the selected read
element is malfunctioning based on a result of the comparing; and
providing an indication when the selected read element is
malfunctioning.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to apparatus and method for
detecting defects in a storage medium and, more particularly, for
detecting defects in a high density storage medium such as a
magnetic disk.
[0003] 2. Description of the Related Art
[0004] Magnetic storage media, such as hard disks or floppy disks,
are comprised of a substrate upon which is coated a thin layer of
magnetic material. Small defects or flaws can exist in the thin
film layer of magnetic material on a disk or the disk substrate,
e.g., pits, plating pin holes, and residual polishing scratches
which cannot be removed by a texturing process. Such defects can
result in writing and reading of erroneous data bits. Such
erroneous bits are created when data is written into a defective
area of the disk and subsequently read out from the disk. A data
bit error for a particular bit is caused either by the
magnetization of a bit being missing or by demagnetization being
added at the storage location.
[0005] In order to test and identify defects in the layer of
magnetic material or disk substrate, a typical prior art technique
used by manufacturers of magnetic media is to perform a surface
analysis of the layer of magnetic material and produce an error map
for the surface of the disk. The error map is then written on the
disk for future reference to avoid the defective areas of the disk
during subsequent recording and playback of data. Alternatively, if
the disk contains an unacceptable number of defects, it may be
wholly rejected.
[0006] Various techniques are known for performing the surface
analysis. One technique for performing the surface analysis
involves writing a test signal such as a high frequency,
alternating data pattern onto the disk. This pattern is then read
from the disk as a high frequency output test signal which has a
sinusoidal waveform consisting of sinusoidal data pulses
corresponding to the recorded data bits of the test signal. The
sinusoidal data pulses are monitored for deviations from the
expected waveform of the pulses to indicate the occurrence of a
defect on the disk. For example, the peaks of the output data
pulses may be compared to a threshold amplitude. If a peak
amplitude of an output pulse is less than the threshold, the
location on the disk corresponding to the pulse is determined to
contain a defect. In accordance with another technique, the phase
of the output test signal is monitored. An output data pulse that
occurs with a shifted phase, i.e., occurs outside an expected time
window of occurrence, may be determined to correspond to a defect
in the disk.
[0007] A summary of such conventional defect testing techniques is
disclosed in U.S. Pat. No. 4,929,894.
[0008] Separately, developments in magnetic recording media
materials and media manufacturing techniques as well as read/write
head and disk drive design, have resulted in continuously
increasing recording capacities. These increasing capacities
correspond to an increase in the recording density of magnetic
media as expressed in recordable bits per unit area, e.g., gigabits
per square inch (Gbits/in..sup.2). For example, recent recording
density requirements have reached 8 Gbits/in..sup.2. With this
increase in recording density, the size of a defect in the layer of
magnetic material that can cause an error has correspondingly
decreased.
[0009] As recording density increases, the time required for a
manufacturer of magnetic media to test the integrity of the media
also increases. The density of data per unit area, i.e., areal
density, for which the media must be capable can be expressed as:
Areal Density=Bit Density.times.Track Density. Thus, manufacturing
throughput may be reduced as a result of increased recording
density. One solution that has been practiced to obviate this
difficulty is to test less than 100% of the magnetic media surface
for defects. The result of this solution is a magnetic disk that
may contain defects that cause data errors during operation. This,
in turn, may cause customer dissatisfaction and decreased
sales.
[0010] Another solution is to use a wider read/write head to test
the integrity of the magnetic medium than would be required to
write and read data with the intended data recording density. This
solution results in a reduced sensitivity to defects during
performance of the surface analysis with the result that smaller
defects may not be detected even though such smaller defects may
nevertheless be sufficiently large to cause read/write errors for
the intended high recording density application. Again, the result
is a magnetic disk that may contain defects that cause data errors
during operation.
[0011] Additionally, malfunctioning of apparatus for testing the
integrity of the magnetic medium may lead to failure to detect
defects or falsely indicating defects. The former case may result
in selling a magnetic disk that causes data errors during operation
and consequent customer dissatisfaction. The latter case may result
in a decision to reject an otherwise acceptable disk.
SUMMARY OF THE INVENTION
[0012] Accordingly, the present invention is directed to apparatus
and method for testing storage media, including magnetic storage
media, that substantially obviates one or more of the problems due
to limitations and disadvantages of the prior art.
[0013] To achieve these and other advantages and in accordance with
the purpose of the invention, as embodied and broadly described
herein, there is provided apparatus for reading a plurality of
information tracks in a magnetic storage medium, comprising: a read
head including a plurality of read elements each producing a read
signal in response to data stored in the tracks of the storage
medium; and an element monitoring circuit to monitor the plurality
of elements and provide an indication when one of the elements
malfunctions.
[0014] Also in accordance with the present invention there is
provided apparatus for reading a plurality of information tracks in
a magnetic storage medium, comprising: a read head including a
plurality of read elements each producing a read signal in response
to data stored in the storage medium; means for storing ones of the
read signals output by a selected one of the read elements; and
means for comparing at least one of the stored read signals with at
least one subsequent read signal output by the selected read
element, for determining whether the selected read element
malfunctions, and for providing an indication when the read element
malfunctions.
[0015] Further in accordance with the present invention there is
provided a method for reading a plurality of information tracks in
a magnetic storage medium, comprising: producing read signals from
a plurality of read elements in response to data stored in the
track of the storage medium; monitoring the plurality of read
elements to determine when any one of the read elements
malfunctions; and providing an indication when any one of the read
elements is determined to be malfunctioning.
[0016] Additionally in accordance with the present invention there
is provided a method for reading a plurality of information tracks
in a magnetic storage medium, comprising: producing read signals
from a plurality of read elements in response to data stored in the
storage medium; storing ones of the read signals output by a
selected one of the read elements; and comparing at least one of
the stored read signals with at least one subsequent read signal
output by the selected read element; determining whether the
selected read element is malfunctioning based on a result of the
comparing; and providing an indication when the selected read
element is malfunctioning.
[0017] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
[0018] The accompanying drawings are included to provide a further
understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1A and 1B illustrate general arrangements of
embodiments of defect detection apparatus consistent with the
present invention
[0020] FIG. 2 schematically illustrates positioning of a certify
head over tracks of a hard disk.
[0021] FIG. 3 diagrammatically illustrates an embodiment of disk
defect detection apparatus consistent with the present
invention.
[0022] FIG. 4 diagrammatically illustrates a multichannel
read/write element monitor consistent with the present
invention.
[0023] FIG. 5 illustrates a possible configuration of a monitor
circuit portion shown in FIG. 4.
[0024] FIG. 6 illustrates a flow chart for controlling radial
movement of a read head.
[0025] FIG. 7a-7f illustrate successive positions of a read head
controlled in accordance with the flow chart in FIG. 6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Embodiments consistent with the present invention overcome
the disadvantages of the prior art by utilizing a read head with
more than one read element to read test signals from multiple
tracks of a recording medium being tested. The multiple signals
respectively generated by the more than one read elements are
processed by one or more certifying circuits to determine whether
any unacceptable defects are present in the storage medium being
tested. The multiple read elements are monitored to detect a
malfunctioning read element and the read head is controlled to
assure a malfunctioning read element is not used for defect
detection.
[0027] FIG. 1A illustrates the general arrangement of a defect
detection apparatus 100 consistent with the present invention.
Apparatus 100 includes a certify head 102 for reading a test
pattern written on a magnetic medium such as a hard disk 104 being
tested for the presence of defects in its layer of magnetic
material and in the underlying substrate. Certify head 102 includes
a read/write head 106 that is positioned over hard disk 104 by a
load beam 108. One end of load beam 108 is attached to head 106 and
its opposite end is attached to a head loading mechanism (HLM) or
cartridge 110. HLM 110 is radially moved inward and outward
relative to hard disk 104 by a high precision linear stepper motor
112. The particular step size resolution required for stepper motor
112 is dictated in part by the recording density of hard disk 104.
For example, for recording densities discussed herein, motor 112
should be capable of effecting step size movements of 0.1 microns
or less.
[0028] With certify head 102 constructed to include read/write head
106, head 102 is used during defect testing both for writing a test
pattern onto hard disk 104 and subsequently reading the test
pattern. Alternatively, certify head 102 can be used for reading
the test pattern and a separate write only head 114 can be provided
only for writing the test pattern. Thus, when write only head 114
is provided, it is not necessary to include write elements on
certify head 102.
[0029] Head 114, when provided, has one end attached to an HLM 116
which is, in turn, radially moved over hard disk 104 by a high
precision linear stepper motor 118.
[0030] A microprocessor 120 is included in apparatus 100 to control
functions performed therein. A memory 122 is coupled to
microprocessor 120 for temporarily storing information regarding
defects detected on the hard disk 104 being tested. One function
controlled by microprocessor 120 is the operation of stepper motor
112 and stepper motor 118. A stepper motor controller 124 is
coupled between motor 112 and microprocessor 120 to facilitate
control of motor 112 operations. Similarly, a stepper motor
controller 126 is coupled between motor 118 and microprocessor
120.
[0031] Microprocessor 120 and memory 122 can be embodied in a
personal computer (PC) with microprocessor 120 coupled to the other
elements of apparatus 100 as shown in FIG. 1A. In such case, a hard
disk drive storage device of the PC can also be used to store the
information regarding defects detected on hard disk 104.
[0032] Motor 112 is controlled by stepper motor controller 124 to
radially move head 106 in discrete steps of a predetermined size
either inward toward the center of hard disk 104 or outward away
from the center. Microprocessor 120 is programmed to control the
operation of controller 124 and move head 106 to successive desired
positions. More particularly, since the step size (SS) of each step
movement caused by motor 112 is known, microprocessor 120 can
determine the radial position of head 106. For example, for
movement of head 106 from the outer periphery of hard disk 104
toward the disk center, and assuming that the radial position of
head 104 has a maximum value of "outer radius" (OR) at the hard
disk periphery, the "current position" (CP) of head 106 is given
by:
CP=OR-number of movements.times.SS.
[0033] Alternatively, if head 106 is moved outward from the center
of hard disk 104 and assuming the innermost radial position of head
106 is IR, the current position of head 106 is given by:
CP=IR+number of movements.times.SS.
[0034] A spindle motor controller 128 controls rotation of a
spindle motor, not shown, for rotating hard disk 104.
Microprocessor 120 is coupled to controller 128 to control the
spindle motor speed of operation and to determine the instantaneous
circumferential position of hard disk 104 during rotation. For this
purpose, the shaft of the spindle motor is preferably monitored to
generate an index signal upon completion of each revolution of hard
disk 104. The circumferential position information is necessary to
enable identifying the physical location of defects on disk 104, as
more fully described below.
[0035] The nature of the test pattern being written and circuitry
for generating such a test pattern are conventional and may be of
the type described above. A read/write preamplifier circuit 130 is
coupled to read/write head 106. Circuit 130 functions as a current
generator to magnetize hard disk 104 through head 106 to write
data, such as a test pattern. Circuit 130 also functions as a
differential amplifier to read data signals from hard disk 104 for
further processing in accordance with embodiments of the
invention.
[0036] A certifier circuit 132 is coupled between circuit 130 and
microprocessor 120. Certifier circuit 132 receives the read signals
from hard disk 104 provided by circuit 130 and evaluates those
signals to detect defects in hard disk 104. Certifier circuit 132
also includes a pattern or frequency generator for generating the
test pattern for writing to hard disk 104 by circuit 130.
Microprocessor 120 is coupled to both circuits 130 and 132 to
control read and write operations. The operation of certifier
circuit 132 is described in greater detail below.
[0037] In the case that write only head 114 is provided, the
read/write preamplifier circuit associated therewith is also
coupled to both microprocessor 120 and certifier circuit 132 to
control writing operations.
[0038] FIG. 1B illustrates apparatus 134 that represents a
variation of apparatus 100, in which a second certify head 140 is
provided, so that apparatus 134 includes a total of two certify
heads. Certify head 140 includes a read/write head 142 positioned
over hard disk 104 by a load beam 144. One end of load beam 144 is
attached to head 142 and its opposite end is attached to an HLM
146. HLM 146 is radially moved inward and outward relative to hard
disk 104 by a high precision linear stepper motor 148. A stepper
motor controller 150 is coupled between motor 148 and
microprocessor 120. A read/write preamplifier circuit 152 is
coupled to read/write head 142 and functions substantially the same
way as circuit 130. Circuit 152 is coupled to microprocessor 120
and certifier circuit 132.
[0039] As described more fully below, when both of certify heads
102 and 140 are provided, defect detection can be simultaneously
conducted by both certify heads to increase the speed of
detection.
[0040] FIG. 2 schematically illustrates in greater detail an
embodiment of certify head 102, including read/write head 106,
consistent with the present invention. With reference to FIG. 2,
read/write head 106 includes a write element 200 and a plurality
of, e.g., four read elements 202, 204, 206, and 208 which are
positioned over and aligned with track areas 210, 212, 214, and
216, respectively, on disk 104 on which have previously been
written test patterns. In the present embodiment, the actual
configuration of read elements 202-208 only requires that the
elements together can be simultaneously positioned over four
adjacent tracks of the disk being tested. Write element 200
preferably has a width, measured across track areas 210-216,
greater than the combined width of read elements 202-208. For
example, write element 200 can be wider than the combined width of
read elements 202-208 by a factor of 1.25-3.0. This insures that
the write element can magnetize the entire area corresponding to
track areas 210-216.
[0041] Read/write head 106 and the read elements thereof are
preferably provided as magnetoresistance effect (MR) elements. MR
elements can have a variety of constructions and are well known in
the art. Examples of the construction of MR elements are disclosed
in U.S. Pat. Nos. 4,071,868, 5,568,335, 5,754,376, and 5,081,554.
The respective read elements 202-208 each have a width sufficiently
small to enable writing to and reading from disk 104 with a density
at least as large as the intended recording density of disk 104. As
a result, read/write head 106 provides a desired sensitivity for
detecting defects in the magnetic layer and/or substrate of disk
104. For example, in accordance with one of the techniques noted
above for detecting defects, the test signal read from the disk is
a series of sinusoidal data pulses. If the amplitude of one of the
sinusoidal pulses of the read signal fails to reach a threshold
value that is a predetermined percentage of an expected amplitude
of the pulses, then the location on the disk corresponding to that
particular pulse is determined to contain a defect. Equation (1)
provides a relationship between a critical defect size (CD)
corresponding to the smallest defect size that is expected to cause
read/write errors for the intended recording density for the
magnetic medium and is, therefore, to be detected, the width of the
read element, and the signal amplitude threshold:
CD=(1-threshold).times.(read element width) (1)
[0042] For recording densities of approximately 8 Gbit/in.sup.2, a
value of CD is approximately 0.2 .mu.m. For recording densities
greater than 8 Gbit/in.sup.2, the value of CD should be decreased
with increasing recording density. As recording density/areal
density increases, the bit density and track density increase,
which causes the space for the same number of recorded bits to
shrink, therefore, CD will become smaller. The value CD=0.2 is
exemplary and does not correspond to a specific recording density.
If the threshold value is set at too high a percentage, e.g. 90%,
the test results may be over-inclusive. This is because a single
pulse having a nominal value only marginally less than or equal to
90% will be treated as corresponding to a defect. However, this
type of 90% threshold rejected defect would not cause a read/write
error in a magnetic medium drive application. Indeed, pulses of
such amplitude may be caused by phenomena unrelated to defects,
e.g., grain size distribution in the magnetic layer. A preferred
threshold value is about 60-75%. Considering, for example, a
threshold value of 75%, based on equation (1), a CD of 0.2 .mu.m
yields a read element width of 0.8 .mu.m.
[0043] It may also be useful to set multiple thresholds to be
applied during testing to, for example, enable the grading of
products.
[0044] Equation (1) also illustrates that by increasing the read
element width, sensitivity to defect size is decreased as may be
done in conventional testing to increase throughput while failing
to detect defects having a size smaller than CD.
[0045] FIG. 3 diagrammatically illustrates an embodiment of disk
defect detection apparatus consistent with the present invention
including multichannel certifier circuit 132 and the connection
thereof to receive the read signals from read/write head 106 after
amplification by circuit 130. For convenience, write element 200 is
not shown. Certifier circuit 132 includes certifier circuit
portions 300, 302, 304, and 306, coupled to circuit 130 to receive
respective amplified read signals provided by read elements 202,
204, 206, and 208 through circuit 130. Each certifier circuit
portion includes signal processing circuitry for evaluating the
read signal from its corresponding read element to detect defects
in the magnetic medium and/or substrate. The signal processing
circuit included in each certifier circuit 300-306 is adapted to
perform defect detection in accordance with any conventional
technique or techniques. For example, each circuit portion 300-306
may be adapted to compare the sinusoidal pulses of a test signal
read from disk 104 to a predetermined threshold, e.g., set at 75%
of full pulse amplitude, and provide a defect detection output for
each pulse having an amplitude less than the threshold.
[0046] The detection results of the certifier circuit portions are
provided to microprocessor 120 which causes them to be stored in
memory 122 or in a storage device such as a hard disk drive of a PC
in which microprocessor 120 is embodied. Microprocessor 120 further
correlates detected errors with their physical locations on hard
disk 104. With such information determined by microprocessor 120,
it can be programmed to provide a report with information of
particular interest to the user. For example, for each defect, the
report can specify the radius at which, and the sector within
which, the defect occurs and the bit length of the defect.
Providing the bit length of the defect further enables each defect
to be characterized as correctable or noncorrectable, i.e., whether
or not the defect will cause a malfunction in the operation of the
drive in which it is ultimately installed.
[0047] In operation, the manner in which writing and subsequent
reading of a test signal depends on whether apparatus 100 is
provided with a single certify head 102 alone or with write only
head 114, or whether apparatus 134 is provided with multiple
certify heads, such as 102 and 140 (FIG. 1B). If single certify
head 102 including its read/write head 106 is provided, apparatus
100 is first operated to write a test signal onto disk 104
utilizing head 106 and subsequently to read the test signal by
again utilizing head 106. The test signal read from disk 104 is
processed to identify defects as further described below.
[0048] If apparatus 100 includes both certify head 102 and write
only head 114, heads 102 and 114 can be positioned at diametrically
opposite positions over disk 104, i.e., positioned 180.degree.
apart, by microprocessor 120 via their respective stepper motor
controllers and stepper motors over the same track at the same
time. Then, head 114 is controlled to write the test signal onto
disk 104 and head 102 is controlled to read that test signal as the
recorded track on disk 104 rotates into position for reading by
head 102. As each track is written and read, the respective heads
102 and 114 are moved by discrete steps to write to and read from
successive tracks. The test signal read from disk 104 is processed
to identify defects. However, when a defect is detected,
microprocessor 120 stops writing of the test signal and the track
containing the apparent defect is re-read to verify the existence
of the defect. After completion of verification, the writing and
reading of successive tracks again commences.
[0049] If apparatus 134 is provided including two certify heads 102
and 140 positioned at diametrically opposite positions over disk
104, as shown in FIG. 1B, the heads are preferably controlled so
that one head is controlled to move stepwise from an innermost
radius to a middle radius to cover an inner radial portion of disk
104, while the other head is controlled to move stepwise from the
outermost radius to the middle radius to cover an outer radial
portion of disk 104. In this manner, heads 102 and 140 are moved
simultaneously so that the inner and outer radial portions are
simultaneously tested for defects. More particularly, first, heads
102 and 140 are moved stepwise under control of microprocessor 120
to record the test signal on disk 104. Then, heads 102 and 140 are
again moved stepwise under control of microprocessor 120 to read
the test signal from disk 104. Since the two heads 102 and 140
simultaneously operate to write to, and read from, disk 104, the
time for conducting the entire operation is substantially
halved.
[0050] Each step movement of HLM 110 when head 102 is provided
alone or HLMs 110 and 146 when both heads 102 and 140 are provided,
is recorded by microprocessor 120 and is converted into a physical
radial position on disk 104 for assessment of disk integrity as
well as for subsequent engineering mode media failure analysis.
With physical radius and circumferential position information, it
is also possible for microprocessor 120 to determine sector
information. For example, hard disk 104 may be divided into 1024
sectors.
[0051] During simultaneously reading of the test signals by read
elements 202-208, the readback signals from all read elements are
analyzed according to the threshold value set in each of certifier
circuit portions 300-306, e.g., 75%. If any bit/pulse of the
readback signal has a magnitude below this threshold, the certifier
circuit will pass this information to the microprocessor 120 and it
will be recorded as a defect, e.g., optionally characterized as a
missing pulse. Microprocessor 120 will also record the location of
the defect, such as by its physical radius and sector. By
accumulating information regarding successive missing pulses,
microprocessor 120 can also record the bit length of a defect.
Thus, microprocessor 120 accumulates the number of defects the
system found during the entire surface analysis of disk 140. Extra
pulse, modulation, and thermal asperity defects can also be
detected by apparatus 100. These defects are defined in the
standards of the International Disk Equipment and Material
Association (IDEMA) located in Sunnnyvale, Calif. The defect
information can be used to categorize the media according to its
grade. The defect information is also useful to create an error map
for further engineering mode failure analysis use. In the process
of evaluating the readback signals, it may be desirable to read the
same track more than once to confirm the existence of defects.
[0052] The defect detection of each track preferably includes a
retry cycle in which the test signal is read again and evaluated to
determine whether any defects are present.
[0053] As defect detection of each track is completed, the
microprocessor controls the certify head and, if present, the write
only head to move by one step radially inward or outward as
appropriate. The step size as noted above is selected according to
the required recording density of the disk but is typically equal
to the combined width of the read elements, e.g., read elements
202-208 of head 106.
[0054] Since read elements 202-208 each have a width that is
sufficiently small to enable detection of critical defects and
since an annular area of disk 104 corresponding to four tracks is
scanned during each revolution of the disk, the testing of disk 104
proceeds at a high rate, with high sensitivity, and covers 100% of
the usable disk surface. However, the user can set the step size
for head movement to provide a different surface coverage, i.e.,
less than 100% coverage.
[0055] FIG. 4 diagrammatically illustrates a multichannel
read/write element monitor circuit 400 consistent with the present
invention and the connection thereof to certify head 102. Monitor
circuit 400 is preferably included within read/write preamplifier
circuit 130 or may be coupled to an output port thereof. Monitor
circuit 400 includes monitor circuit portions 402, 404, 406, and
408 respectively coupled to read elements 202, 204, 206, and 208.
For convenience, write element 200 is not shown. Each monitor
circuit portion 402-408 includes circuitry for monitoring the
integrity of the corresponding read element to which it is
connected and for providing an output signal upon detecting that
its corresponding read element is malfunctioning. Thus, monitors
402, 404, 406, and 408 provide a malfunction signal output on their
respective outputs 410, 412, 414, and 416 when any of read elements
202, 204, 206, and 208, respectively, are detected to be
malfunctioning.
[0056] The type of circuitry provided for each monitor circuit
portion depends on the type of read element being utilized. FIG. 5
illustrates an example of the monitor circuit portion when read
elements are provided as MR elements. With reference to FIG. 5, a
monitor circuit portion 500 includes a resistor 502 connected
across an MR read element 504 being monitored. Circuit portion 500
includes a current source 506 for driving a predetermined current
through element 504 when the element is tested. A voltage V.sub.MR
developed across the parallel combination of element 504 and
resistor 502 is applied to the inverting input of a comparator 508
implemented with an operational amplifier (op-amp). The voltage
V.sub.MR is also applied to the non-inverting input of an op-amp
which serves as a comparator 510. The non-inverting input of
comparator 508 is connected to a first DC reference voltage
V.sub.1. The inverting input of comparator 510 is connected to a
second DC reference voltage V.sub.2. V.sub.1 is selected to have a
value less than V.sub.MR under normal conditions, while V.sub.2 has
a value greater than V.sub.MR under normal conditions. Thus,
V.sub.1<V.sub.2. The respective outputs of comparators 508 and
510 are connected to inputs of OR gate 512 An output OUT of OR gate
512 provides an indication when element 504 is malfunctioning.
[0057] Circuit 500 is configured to detect the malfunction of
element 504 by its failing in either an open circuit or short
circuit condition. In the event of the short circuit condition, the
voltage V.sub.MR drops below V.sub.1 and the logical output of
comparator 508 changes from 0 to 1. In the event of an open circuit
condition, the voltage V.sub.MR increases above V.sub.2 and the
logical output of comparator 510 changes from 0 to 1. As a result,
the value of OUT outputted by OR gate 512 becomes logical 1 when
element 504 fails in either the short circuit or open circuit
condition. By the nature of its operation, circuit 500 cannot be
operated while the read elements of head 102 are actively reading
information. Thus, circuit 500 may be operated during a period of
time when data is only being written to but not read from a disk
being tested or when a disk is not being tested. At a user's
discretion, circuit 500 can be enabled to operate after selected
revolutions of the disk during a test operation, although each such
operation of circuit 500 will add to the total time required to
complete testing of the disk and thereby affect system
throughput.
[0058] In accordance with another embodiment of the invention,
separate circuitry to monitor read elements 202-208 is not
provided. Instead, successive values of the read signal from each
element are compared. When there is a predetermined change in the
read signal value relative to previous values, a malfunction of the
read element is determined to have occurred. For example, the
average signal amplitude for an entire track from a particular read
element can be compared with an arbitrary value set by the user.
Then, if the average signal from the particular element
significantly differs from the arbitrary value, then the particular
element is determined to be defective. The successive values of the
read signal may be stored by microprocessor 120 in memory 122 for
monitoring in accordance with criteria for determining whether the
corresponding element has malfunctioned. In this case, either or
both DC reference voltages V.sub.1, V.sub.2 may be provided by a
programmable reference voltage source. This enables the setting of
the reference voltages as a function of the average signal value of
a previously read track.
[0059] In accordance with a further aspect of the present
invention, upon determining that a read/write element has
malfunctioned, certify head 102 is controlled to continue to read
desired tracks of hard disk 104 without using the malfunctioning
element. For example, in the case that a test signal has been
recorded on 100% of the tracks of hard disk 104, microprocessor 120
controls stepper motor controller 124 to move certify head 102 such
that the remaining functioning read elements are successively
positioned over substantially all tracks to test substantially 100%
of the surface of disk 104.
[0060] FIG. 6 illustrates a flow chart 600 for controlling a radial
movement step size of head 106. Flow chart 600 is based on an
example in which the width of each read element 202-208 in the
radial direction of disk 104 is 0.8 .mu.m and that the read
elements are contiguous in the radial direction. As a result, the
combined width of the four elements 202-208 is 3.2 .mu.m. Further,
for convenience of explanation, elements 202-208 are respectively
referred to as the first through fourth elements.
[0061] FIGS. 7a-7c illustrate three successive positions 700, 702,
and 704, respectively, to which head 106 is moved by microprocessor
120 when element 202, i.e., the first element, has malfunctioned.
In FIG. 7a, head 106 is diagrammatically positioned over tracks
210-216 of hard disk 104. For convenience, the curvature of the
tracks is not shown in FIGS. 7a-7c.
[0062] With reference also to flow chart 600 in FIG. 6, in step
602, the number of elements in head 106 (four in this example), the
read width (RW) of each element (in this example 0.8 .mu.m), and a
radial step size of head 106 to achieve 100% coverage of disk 104
assuming no malfunctioning elements (in this example 3.2 .mu.cm)
are initially established. In step 604, so long as none of read
elements 202-208 malfunction, the step size of 3.2 .mu.m is
maintained. However, if one of read/write elements 202-208
malfunctions, the flow chart proceeds to step 606 where a new step
size is assigned. If element 202, i.e., the first element,
malfunctions, then the step size is changed to 2.4 .mu.m. With
reference to FIG. 7a, when head 106 is in position 700, the
outermost track 210 is not read. With reference to FIG. 7b, servo
controller 122 then moves head 106 by the 2.4 .mu.m step size to
position 702 so that malfunctioning element 202 is positioned over
track 216 that was previously read by element 208 in position 700.
In position 702, elements 204, 206, and 208 read tracks 218, 220,
and 222, respectively. With reference to FIG. 7c, when
microprocessor 120 moves head 106 by 2.4 .mu.m to position 704,
malfunctioning element 202 is positioned over a track 222 that was
previously read by element 208 in position 702. In position 704,
elements 204, 206, and 208 read tracks 224, 226, and 228,
respectively. In this way, although outermost track 210 is never
read, all other tracks of disk 104 are read. As a result, even
though element 202 has malfunctioned, defect detection of disk 104
can still be carried out at a high rate.
[0063] FIGS. 7d-7f illustrate an example in which element 204, the
second element, has malfunctioned. Step 606 of flow chart 600
requires that when the second element has malfunctioned, the step
size successively alternates between 0.8 .mu.m and 3.2 .mu.m. Thus,
with reference to FIG. 7d, when head 106 is in a first position
706, malfunctioning element 204 is positioned over track 212 and
that track is not read. However, elements 202, 206, and 208 read
tracks 210, 214, and 216, respectively. With reference to FIG. 7e,
servo controller 122 then moves head 106 by a 0.8 .mu.m step size
to a second position 708 so that element 202 reads track 212 and
element 208 reads track 218 that was not previously read. With
reference to FIG. 7f, servo controller 122 then moves head 106 by a
3.2 .mu.m step size to a third position 710 so that element 204 is
positioned over track 222 and that track is not read. However,
elements 202, 206, and 208 read tracks 220, 224, and 226,
respectively, that were not previously read. With the step size so
alternated, all tracks of disk 104 are read.
[0064] While embodiments consistent with the present invention have
been described, the invention is not so limited thereby. For
example, while the read and write elements of certify head 102 are
disclosed as preferably being MR elements, the invention is not so
limited. The elements can instead be provided as magnetic inductive
type elements or magnetic-optical sensors, provided the elements
can read and write with a data density commensurate with the
intended density of the disk being tested.
[0065] While certify head 102 has been disclosed as including a
read/write head including four read elements, the invention can be
practiced with equal effectiveness with a read or read/write head
including a number of read elements greater or lesser than four. As
the number and size of the read elements are changed, the step
sizes in flow chart 600 need to be adjusted accordingly.
[0066] It will be apparent to those skilled in the art that various
modifications and variation can be made in the apparatus and the
method of the present invention without departing from the spirit
or scope of the invention. Thus, it is intended that the present
invention cover the modifications and variation of this invention
provided they come within the scope of the appended claims and
their equivalents.
* * * * *